Shallow copy vs deep copy

A shallow copy will clone the top-level object, but nested object are shared between the original and the clone. That's it: an object reference is copied, not cloned. So if the original object contains nested object, then the clone will not be a distinct entity.

A deep copy will recursively clone every objects it encounters. The clone and the original object will not share anything, so the clone will be a fully distinct entity.

Shallow copies are faster than deep copies.

When it is ok to share some data, you may use shallow copy. There are even use case where it is the best way to do the job. But whenever you need to clone a deep and complex data structure, a tree, you will have to perform deep copy. Have in mind that on really big tree, it can be an expensive operation.

By the way, if the property is a getter/setter, then descriptor.value will be undefined, so we won't perform recursion on them, and that's what we want. We actually don't care if the getter return an object or not.

That means that o.loop.a = 'Ha! implies that console.log( o.a ) outputs "Ha!" rather than "a". You remember how object assignment works? o and o.loop simply point to the same object.

However, the naiveDeepCopy() method above does not check that fact and therefore is doomed, iterating o.loop.loop.loop.loop.loop... forever.

That what is called a circular reference.

Even without the loop property, it happens that the original object want that the subcopy and sub properties point to the same object. Here, again, the naiveDeepCopy() method would produce two differents and independent clone.

So... o is an object containing three properties, the first is a string, the two others are methods.

The methods are currently using a variable of the parent scope, in the scope of myConstructor(). That variable (named myPrivateVariable) is created when the constructor is called, however while it is not part of the contructed object in any way, it still remains used by those methods.

Therefore, if we try to clone the object, methods of both the original and the clone will still refer to the same parent's scope variable.

It would not be a problem if this was not a common Javascript's pattern to simulate private members...

As far as I know, there is no way to alter the scope of a closure, so this is a dead-end: pattern using the parent scope cannot be cloned correctly.

Next step: using a library

Okey, so far, we have done a good job hacking Javascript, and it was fun.

Some optimization work have been done, so tree.clone() should be able to clone large structure efficiently.

One big step in optimization: removing recursivity in the algorithm – it's all taking place in a loop. It avoids stack-overflow and function's call overhead. As a side-effect, depth-first search has been replaced by a breadth-first search algorithm.

Great news: this method is able to detect circular references and reconnect them if the circular option is set to true! Oooh yeah!

Nothing unusual, each caller's argument are assigned to a callee's argument just like it would if you had manually used the = operator. There are no special case for object.

When you pass a variable by reference in a language that supports this pass by reference feature, the caller & callee variable are identical, as if they were each others aliases, so mutating one mutates the other.

Here in Javascript, we have two distinct variables, that happen to point to the same object... ... ... until re-assignment happens.

That's why I prefer to say that a variable, after an object assignment, behaves like a pointer to that object. In a C/C++ fashion, object = { a: 1, b: 2 } should be understood as object = &( { a: 1, b: 2 } ).

How to perform a shallow copy of an object in Javascript

Javascript does not have built-in object-cloning facilities.

A quick and dirty way to clone an object would be to create a new empty object, then iterate over the original to copy properties one by one.